There are known sewing machines such as those for industrial use in which various modes of stitching control are automatically effected. It is necessary in such sewing machines that a pulse generator for speed control, that is, a pulse generator for detecting the speed of rotation of an arm shaft of the sewing machine be provided, and detection be made of an upper needle position, a lower needle position, and a timing for cutting off a needle thread. For such detection, there has conventionally been employed a detector comprising a magnetized ring magnet and a Hall-effect element, the ring magnet being secured to the sewing machine arm shaft.
In order to carry out reliable detection, the ring magnet of the prior detector is required to be relatively large in size, particularly thick, and hence the detector per se is of a large size. Due to a large moment of inertia, the detector is disadvantageous in that its rotatable part or magnet tends to be damaged especially when used on high-speed sewing machines which have become available in recent years. Another difficulty with the conventional detector is that the detecting position and the like of the detector cannot easily be adjusted or otherwise changed dependent on the type or mode of use of the sewing machine.